Laminaria saccharina photosynthesis measured in situ: photoinhibition and xanthophyll cycle during a tidal cycle

Photosynthetic activity in thalli of Laminaria saccharina (Lamouroux) was followed in situ in the upper subtidal zone of a Northern Brittany rocky shore (Roscoff, France), using a submersible pulse amplitude modulated (PAM) fluorometer. Two fluorescence parameters, the effective quantum yield of photosystem II (Phi(PSII)) and the relative electron transport rate (rETR), were estimated at various stages of a tidal cycle from 10:00 to 18:30 h, and different light conditions due to variations of water depth and position of the sun. The Phi(PSII) decreased strongly during the ebb tide, essentially due to a drop in the maximal fluorescence level for light-adapted samples (F-m'). This was the result of increasing non-photochemical quenching (NPQ). Algae totally recovered during the rising tide, indicating that no significant photosynthetic damage occurred at ebb tide. L. saccharina responded to high light stress with photoprotective processes such as the xanthophyll cycle. The de-epoxidation ratio (DR) (i.e. conversion of violaxanthin into antheraxanthin and zeaxanthin) increased during the ebb tide and decreased during the rising tide. However, in spite of the development of a photoprotective mechanism, the overall photosynthetic activity (rETR) declined strongly at the highest irradiance level. This result indicates that primary production levels have been overestimated in the past

Spatio-temporal variability of intertidal benthic primary production and respiration in the western part of the Mont Saint-Michel Bay (Western English Channel, France)

Abstract In situ measurements of both community metabolism (primary production and respiration) and PAM fluorometry were conducted during emersion on intertidal sediments in the Mont Saint-Michel Bay, in areas where oysters and mussels were cultivated. Results highlighted a low benthic metabolism compared to other intertidal areas previously investigated with the same methods. Comparisons between gross community primary production and relative electron transport rates confirmed this statement. More specifically, primary productivity remained very low all over the year, whereas the associated microalgal biomass was estimated to be high. We suggest that the microphytobenthic community studied was characterized by a self-limitation of its primary productivity by its own biomass, as previously shown in Marennes-Oléron Bay for example. The almost permanent high biomass would represent a limiting factor for micromigration processes within the first millimetres of the sediment. This could be explained by very low resuspension processes occurring in the western part of the bay, enhanced by the occurrence of numerous aquaculture structures that could decrease tidal currents in the benthic boundary layer

The Peculiar Features of Non-Photochemical Fluorescence Quenching in Diatoms and Brown Algae

International audienceDiatoms and brown algae are major contributors to marine primary production. They arebiologically diverse, with thousands of different species, and are extremely successful, occupyingalmost every marine ecosystem ranging from the coastal-estuarine to deep-sea regions.Their ecological success is based in part on their ability to rapidly regulate photosynthesis inresponse to pronounced fluctuations in their natural light environment. Regulation of light excessive energy as heat. Thermal dissipation of excitation energy is assessed as non-photochemicalquenching of chlorophyll a fluorescence (NPQ). NPQ depends strongly on theconversion of xanthophylls: diadinoxanthin (Dd) to diatoxanthin (Dt) in the Dd-Dt cycle ofdiatoms and violaxanthin (V) to zeaxanthin (Z), via the intermediate antheraxanthin (A), inthe VAZ cycle present in brown algae. Xanthophyll cycle (XC)-dependent thermal energydissipation underlying NPQ represents one of the most important photoprotection mechanismsof diatoms and brown algae. In the present chapter, we review the biochemistry of XCenzymes with a special focus on co-substrate requirements and regulation of enzyme activity.In addition, we present a new model for the structural basis of XC-dependent NPQ indiatoms based on the latest experimental findings. In the last section, we highlight the importanceof XC-dependent photoprotection for the ecological success of diatoms and brownalgae in their natural environments